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F-box protein 10, a Novel Anti-Apoptotic Protein, Represses c-Fos and Regulates TRAIL-induced Apoptosis
Rongbin Ge, M.D., PhD, Zongwei Wang, M.D., PhD, Aria F. Olumi, M.D..
Massachusetts General Hospital, Boston, MA, USA.

BACKGROUND: Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) induces selective apoptotic death of human cancer cells while sparing normal human cells. Although TRAIL holds great promise as a potential chemotherapeutic agent, multiple tumor types are resistant to this agent. F-box protein 10 (FBXL10), the largest member of the human family of F-box proteins, is a substrate-recognition component of SCF ubiquitin-ligase complexes and transcriptional repressor of AP-1 family member proteins. However, the role of FBXL10 in apoptosis is poorly understood. Previously, we have demonstrated that the AP-1 protein, c-Fos, represses the anti-apoptotic molecule, c-FLIP(L), in order to prime cancer cells for TRAL-induced apoptosis. Here, we report a novel anti-apoptotic function of FBXL10 and show that downregulation of FBXL10 is essential to sensitize cancer cell to TRAIL-induced apoptosis through upregulation of c-Fos transcriptional activity in vitro and in vivo.
METHODS: FBXL10 mRNA and protein levels were assayed by RT-PCR and Western blot in TRAIL sensitive and TRAIL resistant cancer cells. FBXL10 expression was repressed by RNAi. Co-transfection of FBXL10 cDNA and c-Fos promoter plasmid and chromatin immunoprecipitation were performed to analyze the interaction between FBXL10 protein and c-Fos promoter. Finally, in orthotopic in vivo prostate xenografts, FBXL10 protein expression was assessed by immunofluorescence staining.
RESULTS: In TRAIL sensitive cancer cells, FBXL10 RNA and protein levels significantly decreased after treatment with TRAIL in a dose and time-dependent manner whereas c-Fos was upregulated in an inverse relationship. However, in TRAIL resistant cancer cells, FBXL10 and c-Fos levels were not affected. Expression level of FBXL11, a FBXL10 homologous protein, was not affected by TRAIL treatment. Silencing of FBXL10 by RNAi in TRAIL-resistant cancer cells induced upregulation of c-Fos and primed the cancer cells to undergo apoptosis. Moreover, co-transfection of FBXL10 and human c-Fos promoter repressed c-Fos promoter activity. However, FBXL10 did not repress c-Fos promoter activity when AP-1 binding sites on the c-Fos promoter were mutated. Furthermore, using the ChIP assay, we observed that FBXL10 protein directly binds the c-Fos promoter in the TRAIL sensitive cells, and after treatment with TRAIL the interaction between FBXL10 and c-Fos promoter is significantly reduced. Finally, in the orthotopic in vivo prostate cancer xenografts treated by a TRAIL receptor 2 agonist antibody, (Lexatumumab), significant downregulation of FBXL10 immunofluorescence was observed in the TRAIL sensitive xenografts, but not in the TRAIL resistant xenografts, further establishing the role of FBXL10 in TRAIL-induced apoptosis.
CONCLUSIONS: We demonstrate that the FBXL10 protein has a novel anti-apoptotic function in regulating TRAIL induced apoptosis. c-Fos, which is necessary for priming prostate cancer cells to undergo apoptosis, is repressed by FBXL10 in TRAIL resistant prostate cancer cells. Differentiating molecular mechanisms between TRAIL sensitive and TRAIL resistant cancer cells will improve the efficacy of apoptotic therapies for prostate cancer patients.


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